1. (Field of the Invention)
The present invention relates to a device for measuring oxygen concentration in industrial applications purposes. More specifically, the invention relates to an oxygen concentration measuring device suitable for use in the measurement of oxygen concentration in the atmosphere and also for use in the measurement of oxygen concentration in a combustion exhaust gas of an industrial furnace or a boiler by inserting the device into the furnace or in the exhaust gas passageway or affixing it on the wall of the furnace.
2. (Related Art Statement)
A conventional device of this kind has been disclosed in Japanese Patent Application Opened Number 54-99492. In order to help better understand this invention, such a known device will be explained by referring to FIG. 1.
In this measuring device, the sensing cell is formed by an oxygen concentration cell 1. The oxygen concentration cell 1 comprises a closed bottom cylindrical type solid electrolyte element 2, and an inner electrode 3 and an outer electrode 4 provided on the inner and outer surfaces, respectively, of the closed bottom end of the solid electrolyte element 2. Said closed end is placed in a gas passageway 6 by holding the solid electrolyte element 2 by a holder 5 detachably coupled to a frame 7 of the measuring device and an open end of the solid electrolyte element 2 is placed in the open air atmosphere. Lead wires 8 and 9 are coupled to the inner and outer electrodes 3 and 4 to derive out the induced voltage therebetween.
Around the outer surface of the closed end of said solid electrolyte element 2, heater 10 is arranged so as to heat the oxygen concentration cell 1 up to about 500.degree. C.-1000.degree. C. This heater 10 is provided in the gas passageway 6 of the frame 7 in a gas-tight manner. A probe 12 is provided to introduce the combustion exhaust gas to be measured into the gas passageway by suction action of ejector nozzle 11. At the top end of the probe 12, a dust filter 13 is provided. Furthermore, a heater tube 14 mounted with the heater 10 around it is provided and a heat insulating layer 15 is provided to surround the heater tube 14.
An inlet tube 16 of calibration gas is provided and an exhaust passageway 17 and another heat insulating layer 18 are provided.
This known device has disadvantages in that the sensing portion comprising the sensing cell formed of the oxygen concentration cell 1 and a heater member. The heat member has a heater 10 and a heater tube 14 and includes other components of a considerably large size and thus the thermal capacity becomes relatively large. Therefore, a long time is required before the sensing cell of the oxygen concentration cell 1 assumes a predetermined working temperature by the heating of a heat source, for instance by the heater 10. Namely, the warming-up time counted from the start of heating of the heater 10 to the commencement of the actual measurement is very long and furthermore the response speed is slow since the sensing portion is of a large size.
Another oxygen concentration measuring device is known as shown in FIG. 2, which measures the oxygen concentration in an exhaust gas and in which the gas to be measured is introduced in an oxygen concentration detector 1 through an aid of suction of the air ejector 72.
This known oxygen concentration measuring device is of an industrial use and to be used by mounting it on the furnace wall of a combustion furnace etc. and it comprises a main flow tube 73 into which the exhaust gas to be measured for its oxygen concentration and taken by a probe 71 is sucked by suction of an air flow ejected from an air ejector 72, a branch tube 74 through which a branched exhaust gas stream is returned back to the main flow tube 73, an oxygen concentration sensor 75 comprising a closed bottom cylindrical solid electrolyte element 75 detachably mounted at the branch tube 74, a heating furnace 76 fixed near the outer surface of the oxygen concentration sensor 75 and a calibration gas outlet tube 77 connected to a branch tube wall of the gas flow inlet side to the oxygen concentration detector 75.
However, the abovementioned industrial oxygen concentration measuring device has disadvantages also in that the size and weight become large and hence the thermal capacitance increases which results in a long warm-up time due to the fact that the oxygen concentration sensor 75 has a closed bottom cylindrical shape and that the heating furnace 76 is provided separately from the detector 75. Furthermore, as the exhaust gas flows in the branch tube by thermal convection so that the response characteristics are insufficient and the mounting position of the measuring device is limited due to the large-size of the detector.
As an oxygen concentration measuring device using an air ejector, a device having a construction as shown in FIG. 3 is also known, in which the top of the probe 71 is inserted into the exhaust gas passageway 79 from a furnace wall 78, a measuring gas passage 80 is provided at the base portion of the probe 71 and an oxygen sensor 75 having closed bottom cylindrical zirconia electrolyte member and an ejector 72 are provided in this measuring gas passageway.
In the aforementioned industrial gas or oxygen concentration measuring device, as the measuring gas is introduced by a probe 71 into the oxygen sensor 75 there were disadvantages in that the response time varies according to the variation of flow rate of the measuring gas and that the time required for the measurement is long.
Furthermore, as the measuring gas passageway 80 and the oxygen sensor 75 are provided outside of the furnace wall 78, which has a relatively low temperature, the temperature of the measuring gas sucked by the ejection stream of the ejector 72 becomes low. This causes condensation of water (H.sub.2 O) content and acid (SO.sub.x for instance SO.sub.2 etc) content included in the exhaust gas which causes corrosion of the corresponding parts and also causes blocking of the tube by catching the dust in the exhaust gas.
A further industrial oxygen concentration measuring device for measuring the oxygen concentration in an exhaust gas collected by a probe by using a solid electrolyte element, is disclosed in Japanese patent application opened specification No. 56-69553. Namely, such a device is as shown in FIG. 4. This oxygen concentration measuring device comprises an oxygen concentration sensing element 1 formed of a closed bottom cylindrical solid electrolyte element combined with a gas collector 85 having a gas flow inlet 83 and a gas flow outlet 84 arranged to surround a gas flow buffer plate 82 provided at an entrance side of a filter 81, which is also arranged to surround said closed end of the solid electrolyte element.
However, the abovementioned oxygen concentration detector uses a Nichrome wire heater and a closed bottom oxygen sensing element so that about 50 mm diameter is required for the probe and the overall device size becomes large and thus the weight becomes heavy. Additionally, this detector also has inconveniences in that the response time is considerably long and the applicable temperature range and range of measuring concentration are narrow.
In the known oxygen concentration measuring device for detecting oxygen concentration in the exhaust gas of a boiler of a steam power station or various combustion furnaces, it is the usual practice to form the measuring devices separately for the high temperature purpose and for the low temperature purpose making about 600.degree. C. as the boundary therebetween.
In the measuring device for the high temperature purpose to be used for temperature higher than about 600.degree. C., as the temperature is already in the working temperature or above it, a temperature measuring element for measuring the temperature of the oxygen sensor element in the proximity of the sensing element and the electromotive force of the oxygen sensing element at each measuring temperature is calibrated at first and then it is derived as an output signal.
In the measuring device to be used at a temperature equal to or lower than about 600.degree. C. to the room temperature, as the above temperature is, in general, lower than the working temperature of the oxygen sensing element, the temperature in the proximity of the oxygen sensing element is adjusted by a heater to be above the working temperature, and the electromotive force E corresponding to the oxygen partial pressure in the measuring gas is processed by an exponential logarithmic conversion circuit in the analysis unit and the oxygen concentration is derived at the output as a directly readable value.
In these kind of the known oxygen concentration measuring devices having separate arrangements for high temperature use and for low temperature use, it was very troublesome to exchange the above two kinds of measuring devices to meet the respective measuring temperature range when the oxygen concentration is to be measured in an atmosphere varying its temperature from room temperature to about 1400.degree. C.
In the above kinds of known measuring devices, a small measuring chamber is arranged for mounting the probe for collecting the measuring gas. The construction of such a measuring chamber will be explained by referring to FIG. 5.
The closed bottom cylindrical solid electrolyte element 2 is provided with an inner and outer electrodes 3, 4 which are formed by coating precious metal such as platinum on both inner and outer surfaces of said solid electrolyte element and firing said coating layers. The solid electrolyte element 2 with electrode layers 3, 4 are housed in a metal cylindrical tube 86 being supported to keep the position in the cylinder by closely stuffing ceramic fibers 87 made of metal oxide fibers such as alumina, silica, etc. A lead wire protecting tube 88 is provided to passing the lead wires to the inner an outer electrodes 3 and 4 for protection.
A reference or standard air inlet tube 89 for introducing the reference air, for instance the atmospheric air, is provided at inner side of the closed bottom cylinder of the solid electrolyte element 2. The reference air inlet tube is supported by a fixing member 90. In the reference air inlet tube 89 a temperature measuring element 91 for instance formed as a platinum and platinum-rhodium thermocouple is inserted to measure the temperature of the solid electrolyte element.
At an open end of said metal cylindrical tube 86, a ceramic filter 13 is provided allowing to pass the measuring gas by diffusion. A calibration gas inlet tube 16 for introducing said reference or calibration gas to contact with the outer electrode 72 of the solid electrolyte element 71 and an exhaust outlet 92 for exhausting the measuring and the calibration gases are provided on the cylindrical tube 86.
This device has also disadvantages in that the closed bottom cylindrical solid electrolyte element 2 for calibrating the sensing portion becomes a large size especially due to a large size of the oxygen sensing portion at the closed bottom from the construction. For the above reason, it has been very difficult or almost impossible to minimize the size of the small measuring chamber A housing the oxygen sensing portion D and being bounded by said cylindrical tube 86 and the ceramic fiber filter 87 to be smaller than a certain limit.
For this reason, for the calibration of the device, a comparatively large amount of the calibration gas should be introduced into the small measuring chamber A and also as the supply time thereof is long and thus the calibration requires a comparatively long time. This causes a problem in that the calibration is not carried out in a simple manner during the measurement.
In the abovementioned known oxygen concentration measuring devices, it was necessary to derive the electric signal generated in the oxygen detecting element out of the furnace. For this purpose, auxiliary lead wires for connecting the oxygen sensing element to the outside lead wires are provided in the probe.
One example of the lead wire mounting structure is shown in FIG. 6. This lead wire mounting structure comprises a protecting tube 93 housing the oxygen sensing element inside, a flange 94 mounted on the protecting tube 93, lead wires 95 connected to the connecting terminals provided at the end of oxygen sensing element and for deriving out the output signal from the oxygen sensing element or for supplying control signals thereto, and an intermediate connecting terminals 96 made of plastics for connecting to the lead wires 95.
The electric connection between the oxygen concentration detecting element and the auxiliary lead wires may be acceptable for instance by a connector joint having spring pieces made of phosphor bronze etc. in a low temperature range. However, in a high temperature range, such connecting means using spring contact are quite unsatisfactory since the spring features thereof are deteriorated and may cause bad contact in the electric circuit. For this reason, in the conventional devices used for the high temperature range, the electric contacts of the oxygen detecting element are soldered to platinum lead wires by means of silver solder or platinum solder. This makes the electric connection itself quite satisfactory. But the connecting work is rather troublesome. Furthermore, when the oxygen sensing element is provided at the top of the probe like in the direct insertion system, the exchange of the oxygen sensing element inevitably is accomplished by the exchange of the whole probe and this is uneconomical.
In the device as mentioned above, as the lead wires 95 are exposed to the surroundings at location between the protecting tube 93 and the intermediate connecting terminal 96, there will be a danger of brake down or melting of the lead wires 95 by the contact to other devices such as the combustion furnace due to flexibility of the same.